The phenomena colloquially known as “platelet swirl” has been known and observed by clinicians for a long time. For instance, it is known that platelet swirl can be observed when platelets in a conventional or standard platelet storage bag are moved in the bag and held against a light source. Many people are looking for swirl in platelet products, but such human observation is very subjective and there is no known basis for analytically assessing or measuring platelet swirl. Nonetheless, it is known that the swirl goes away during aging or when platelets get activated by bacteria or other stress factors such as pathogen inactivation. Swirl might also be absent in a fresh platelet product due to donor factors.
When normal discoid platelets are gently rocked, they scatter the incident light in different directions. Thus, the visually apparent phenomenon known as swirling stems from the moving opalescence caused by the changing orientation of platelets relative to the incident light. When platelets have undergone shape change, which can be considered as a disk-to-sphere transformation, the platelets are said to be activated and they lose the ability to change their orientation. Pseudopods, which are protrusions from the spherical cell body caused by cell activation, do not affect the inability of the cells to demonstrate swirling. As a consequence, all activated platelets scatter light in the same direction, resulting in a dull, unchanging appearance to the sample that is visually distinguishable from swirling. Thus, discoid platelets (and also other nonspherical shapes) show the swirling effect but spherical platelets do not. The observation of platelet swirling is a simple but subjective inherently unreliable test for the nonspherical shape of platelets in concentrates, and thus a subjective assessment of platelet quality.
Although the phenomena of platelet swirl are not well understood and the physical and chemical basis for the phenomena is not well studied, swirl has been used as a quick platelet quality test for many years. Thus, clinicians have often characterized the quality of platelets based upon swirl: if the platelets demonstrate swirl when the bag is manipulated the concentrate may be deemed to be adequate for clinical uses such as transfusion. If the platelets do not swirl, the platelets have been activated and the concentrate might be discarded.
But it will be appreciated that quality determinations based on subjective observations make for a crude and unreliable quality test, especially for a valuable medical product. Nonetheless, there are no known quantifiable tests to assess platelet quality based on swirl; see, e.g., Past and Future Approaches to Assess the Quality of Platelets for Transfusion, Maurer-Spurej, Elisabeth, and Chipperfield, Kate, Transfusion Medicine Reviews, Vol. 21, No. 4 (October), 2007, pages 295-306.
As an illustration of current practice, visual inspection of the swirling effect might be performed before a platelet concentrate is released for transfusion, yet there is little published evidence linking the observed findings to clinical outcome. A swirling score is sometimes recorded during research studies by extensively trained research personnel but because there is no reasonably objective basis for assigning a score swirl assessments are not used routinely.
However, use of an automated and objective device for routine platelet monitoring of platelet units based on the swirling effect has so far not been particularly successful. The so-called Blood Monitoring System invented by Bellhouse (U.S. Pat. No. 4,675,019) detected light transmission changes in agitated platelet bags. However, this method only detects changes in the surface area of the cells facing the light source. Due to the very small size of platelets (2-3 micrometer diameter (see FIGS. 12 and 13), very small changes lead to highly variable results of low precision. Increasing the precision by using imaging techniques is limited by the small size of platelets, their low density which causes constant thermal movement in suspension and thus low image resolution, and the heterogeneity of platelet concentrates. As a result, no automated test is currently known to routinely measure platelet quality. Currently, the short, 5-day shelf life of platelet concentrates is largely dictated by the risk associated with bacterial contamination and not by platelet quality. With the implementation of bacterial testing and pathogen inactivation, platelet quality will become the major determinant for the shelf life of platelet concentrates. However, extended use of platelet concentrates stored beyond 5 days requires quality testing to ensure that the platelet concentrate is suitable for clinical uses such as transfusion. In addition, high platelet quality would be expected to result in improved clinical efficacy, determined by count increment, improved hemostasis, and lower risk for adverse reactions in recipients. No in vitro quality test has yet demonstrated a good correlation with clinical efficacy or improved hemostasis.
There is a pronounced need therefore for an apparatus that facilitates quantitative assessment of platelet quality, and more particularly, the phenomena of platelet swirl, as a measure of the quality of the platelets.